This invention relates to certain hydrocarbon gels and to the use thereof in the drilling of subterranean holes. More particularly, the invention relates to hydrocarbon-based gels containing water absorbent polymers and to the use of such gels in reducing the loss of circulation fluid and in other applications during the drilling of subterranean holes and particularly during the drilling of hydrocarbon wells.
Drilling fluids or drilling muds as they are commonly called are essential materials in the drilling of subterranean holes through rock formations, e.g., in the drilling of hydrocarbon or water wells or in obtaining cores in the evaluation of mineral deposits. Such fluids are used to carry cuttings from the drill bit up the annulus to the surface where the cuttings are removed and the mud reused. The muds also serve to seal small pores within the formation being drilled, to reduce the friction of the drilling operation and to clean and cool the drilling bit. Viscous muds are also used during a pause in the drilling operation to "sweep" the space between the bore hole wall and the drill pipe. Such drilling fluids which, with other materials, are circulated between the bore hole wall and the drill pipe are termed circulation fluids, and are often slurries of clay or other inorganic solids in an aqueous- or hydrocarbon-based fluid.
However, for the drilling or circulation fluid to perform these functions it must remain within the bore hole and not be lost into the formation being drilled. In addition, of course, any loss of circulation fluid is of disadvantage economically. Undesirable loss of circulation fluid is commonly encountered and results from passage of the fluid through fractures in the formation caused by excessive mud pressures or pre-existing pores or fissures which are inherent in the nature of the rock formation being drilled.
There are a number of methods which have been proposed to help prevent the loss of circulation fluid. Some of these methods can be visualized as the use of fibrous, flaky or granular material designed to "plug" the openings through which circulation fluid is lost by causing the particulate material to settle out of the slurry fluid at the entrances to the openings in the formation after passing down through the well bore. Other methods are proposed to use materials which interact in the fissures of the formation to form a plug of increased strength. U.S. Pat. No. 2,890,169 discloses a method in which a circulation fluid is produced from a slurry of bentonite and cement in oil. This fluid is employed in conjunction with a water-in-oil emulsion designed to break within the formation to provide water to the cement with a resulting increase in strength after placement of the formation to provide water to the cement with a resulting increase in strength after placement of the cement within a formation fissure. U.S. Pat. No. 3,448,800 discloses a similar method of reducing the loss of circulation fluid by consecutively passing into the well dissimilar slurries which mix within the formation to form a cement-like plug.
More recently, methods have been available which utilize the ability of certain organic polymers to absorb large quantities of water relative to the volume or quantity of the polymer. Such water absorbent polymers, themselves water insoluble as well as hydrocarbon insoluble, are injected into the well with the objective of encountering naturally occurring or added water at the entrance to and within an opening in the formation. The resulting swelling of the polymer serves to form a barrier to the continued passage of circulation fluid through that opening into the formation. U.S. Pat. Nos. 3,909,421 and 4,128,528 are illustrative of such use of polymers in the reduction of circulation fluid loss.
It will be apparent that the more effective a water-absorbing polymer is, the more difficult the use of such a polymer becomes. U.S. Pat. Nos. 4,124,748 and 4,320,040 disclose polymers shown to be effective in absorbing relatively large quantities of water and thereby substantially expand in volume. However, the placement of such polymers at specific and desired locations is relatively difficult because premature contact with even small amounts of water results in swelling of the polymer at undesirable locations within the bore hole and makes the pumping of the circulation fluid more difficult. Walker, U.S. Pat. No. 4,635,726, describes a method of employing a slug of fluid hydrocarbon having a water absorbant polymer dispersed therein, by injecting the slurry into a borehole and subsequently injecting water. The hydrocarbon slug is forced into a fissure in the formation where it encounters the subsequently added water, provided as such or in the form of an aqueous drilling fluid. The resulting swelling of the polymer serves to plug the fissure and reduce the loss of circulation fluid. In the method of U.S. Pat. No. 4,635,726 the hydrocarbon slug comprises a dispersion of the oleophobic polymer within the hydrocarbon. In using such a slurry, continuous agitation of large amounts of slurry is required to maintain the polymer in suspension and prevent premature settling of the polymer from the hydrocarbon dispersion at undesirable locations. Dymond et al, U.S. Pat. No. 4,670,501 employ polymers which are water-swollen, non-crosslinked polymers to viscosify water-immiscible liquids such as hydrocarbon liquids. The resulting emulsions are said to be useful in drilling operations as lost circulation fluids and fracturing fluids as well as in pipeline pigging operations. It would be of advantage to provide a composition containing hydrocarbon and water absorbing polymer which is more resistant to separation and whose placement and swelling at desired locations during the drilling of a hydrocarbon well is more easily controlled. It would also be of advantage to provide such compositions which, by virtue of controlled swelling upon contact with water, are useful as fluids for viscous sweeps of a subterranean well bore.